In the oil and gas exploration and production industry, wellbore fluids comprising oil and/or gas are recovered to surface through a wellbore which is drilled from surface. The wellbore is lined with metal wellbore-lining tubing, which is known in the industry as ‘casing’. The casing serves numerous purposes, including: supporting the drilled rock formations; preventing undesired ingress/egress of fluid; and providing a pathway through which further tubing and downhole tools can pass.
The casing comprises sections of tubing which are coupled together end-to-end. Typically, the wellbore is drilled to a first depth and a casing of a first diameter installed in the drilled wellbore. The casing extends along the length of the drilled wellbore to surface, where it terminates in a wellhead assembly. The casing is sealed in place by pumping ‘cement’ down the casing, which flows out of the bottom of the casing and along the annular region defined between the external surface of the casing and the internal surface of the drilled wellbore.
Following appropriate testing, the wellbore is normally extended to a second depth, by drilling a smaller diameter extension of the wellbore through a cement plug at the bottom of the first, larger diameter wellbore section. A smaller diameter second casing is then installed in the extended portion of the wellbore, extending up through the first casing to the wellhead. The second casing is then also cemented in place. This process is repeated as necessary, until the wellbore has been extended to a desired depth, from which access to a rock formation containing hydrocarbons (oil and/or gas) can be achieved. Frequently a wellbore-lining tubing is located in the wellbore which does not extend to the wellhead, but is tied into and suspended (or ‘hung’) from the preceding casing section. This tubing is typically referred to in the industry as a ‘liner’. The liner is similarly cemented in place within the drilled wellbore.
Where a liner is employed, a sealing device known as packer is provided at the top of the liner, at the interface with the casing. A packer of this type is usually referred to in the industry as a ‘liner-top packer’. The packer seals the annular region defined between an external wall of the liner, an internal wall of the larger diameter casing that the liner is located in, and the upper surface of cement that has been supplied into the wellbore to seal the liner. The packer may be carried by the liner or deployed independently, and includes a sealing element which can be deformed radially outwardly into sealing abutment with the wall of the casing. Deformation of the sealing element is typically achieved mechanically, for example by axially compressing the sealing element, by setting a certain amount of ‘weight’ on the packer.
Obtaining verification that the packer has been correctly mechanically set, and so provides an adequate seal, is difficult. In the past, the only way of assessing whether a packer had been correctly set was to monitor the weight applied to the packer at surface, that is the axial load imparted upon the packer to urge the sealing element radially outwardly. However, the weight observed at surface often does not correspond to that experienced by the packer, which may be positioned many hundreds of meters downhole. This is a particular problem in deviated wellbores, where it is difficult to apply the necessary weight to set the packer. The only indication that a packer had not been set correctly was if an unexpected leak/pressure drop was detected at surface, such as when pressure testing the liner to check for pressure integrity. Similar difficulties have also been encountered in other steps in wellbore construction activities, where data relating to the activity in question is difficult to obtain.
International Patent Publication No. WO-2010/079327 discloses an apparatus and method to provide monitoring of pressure outside the wellbore casing of a well in which a wireless sensor unit is placed externally of a section of non-magnetic casing, and an internal sensor energizer unit is provided inside the wellbore casing. The sensor unit includes sensors for measuring the pressure and/or temperature of the surroundings, and the sensor unit and energizer unit communicate using electromagnetic modulation techniques. The apparatus disclosed in WO-2010/079327 suffers from the disadvantage that it requires an electric cable extending to surface, both for powering the energizer unit, and for transmitting the measured pressure and/or temperature to surface. This presents significant challenges, in particular: the requirement to store long lengths of cable at surface, where space can be at a premium (especially in offshore environments); the requirement to run the cable through a wellhead at surface and along the length of the wellbore; the requirement to securely couple the cable to tubing deployed into the well; and that there is a significant risk of damage to the cable through contact with downhole components or tubing.